(1) The medaka, Oryzias latipes has been demonstrated to have an XX female/XY male sex-determination system by the sex-linked inheritance of body color as well as by the progeny test of the artificially sex-reversed fish. It is still an open question, however, either the X or the Y has a definitive role in the sex determination process of this fish. In the present experiment, triploid fish were produced, and the relationship between the sex choromosome composition and the sex of the fish were examined. All XXX fish obtained were female, whereas XXY were male, suggesting that the gene (s) on the Y determines the sex differentiation of the fish into male (dominant-Y system).(2) The sex-determination system of Oryzias fishes except Oryzias latipes has not been clarified so far. We produced gynogenetic diploid fish of O.curvinotus and examined their sex from the histological observations on the gonads of juvenile fish. We obtained 13 gynogenetic homozygous fish, all of which were female, i
… Morendication that O.curvinotus also has an XX*-XY* sex-determination system.(3) We examined the sex of the interspecific hybrids between inbred strains of O.curvinotus and O.latipes (HdrR and HNI strains, the former originated from the Southern Population and the latter from the Northern Population). All the interspecific hybrids between HdrR strain and O.curvinotus (X^<HdrR> Y^<O.curvlnotus>) were female fish develop ovaries. On the other hand, the cross between HNI strain and O.curvinotus yields offsprings of both sex, that is X^<HN1> Y^<O.curvinotus> were male. These results can be interpreted that the sex determining gene on Y chromosome (SDY) need some additional genetic factor (s) to determine the sex of the individual, and the SDY of O.curvinotus is incompatible to the factors of HdrR whereas compatible to those of HNI.We conducted genetic analysis between HNI and HdrR to identify the factors. F1 fish between HNI and HdrR were mated to O.curvinotus, and all the interspecific hybrid from these cross were female. Then, the fish from the cross between F1 fish and HNI were mated with O.curvinotus. All the offsprings were also female. These results indicates that not a few genetic factors can be involved in the sex determination of Oryzias fishes. Further genetic analysis is in progress.以上の結果から、メダカ属魚類は哺乳類と同様の性決定機構を持っていること、すなわち、Y染色体上に性決定遺伝子を有し、その発現で個体の性をオスに決めていること、さらに、その性決定過程には複数の常染色体上の遺伝子が関与している可能性が高いことが示された。 Less